Allows you to share a base connection pool, and reuse the pool in different namespaced use cases. (Caution Heroku biased)

application.rb

#config/application.rb
...
    #sets the DB_POOL env variable so that we can use it when setting up redis and db in the worker server 
    if Sidekiq.server?
      concurrency = ENV['WORKER_CONCURRENCY'] || Sidekiq.options[:concurrency]
      sidekiq_admin_threads = 2
      poolSize = concurrency + sidekiq_admin_threads
      p "setting ENV['DB_POOL']=#{poolSize}"
      ENV['DB_POOL'] = poolSize.to_s
    end
...

database_connection.rb

#config/initializers/database_connection.rb
Rails.application.config.after_initialize do
  ActiveRecord::Base.connection_pool.disconnect!

  ActiveSupport.on_load(:active_record) do
    if Rails.application.config.database_configuration
      config = Rails.application.config.database_configuration[Rails.env]
      config['reaping_frequency'] = ENV['DB_REAP_FREQ'] || 15 # seconds
      config['pool']              = ENV['DB_POOL'] || ENV['MAX_THREADS'] || 7
      puts "Running database_connection.rb: #{config}"
      ActiveRecord::Base.establish_connection(config)
    end
  end
end

decorated_connection_pool.rb

class ConnectionPool::DecoratedConnectionPool < ::ConnectionPool
  def initialize(parent_pool, &block)
    super({}) {nil}
    @pool = parent_pool
    @checkout_block = block
    @decorated = {}
  end

  def checkout(options = {})
    raw = @pool.checkout
    @decorated[raw.object_id] ||= @checkout_block.call(raw)
    @decorated[raw.object_id]
  end

  def checkin
    @pool.checkin
    nil
  end

  def shutdown
    @pool.shutdown
    @decorated.clear
  end
end

redis.rb

#config/initializers/redis.rb
url = URI.parse(ENV["REDIS_URL"] || 'redis://localhost:6379/1')
poolSize = ENV['DB_POOL'] || ENV['MAX_THREADS'] || 7
p "Redis PoolSize: #{poolSize}"
$redis = ConnectionPool.new(size: poolSize, timeout: 3) {Redis.new(url: url)}

sidekiq.rb

#config/initializers/sidekiq.rb
Sidekiq.configure_client do |config|
  config.redis = ConnectionPool::DecoratedConnectionPool.new($redis) {|c| Redis::Namespace.new('ns', :redis => c)}
end

Sidekiq.configure_server do |config|
  config.redis = ConnectionPool::DecoratedConnectionPool.new($redis) {|c| Redis::Namespace.new('ns', :redis => c)}
end